There are reflectors in which a reflection film containing silver is formed on a substrate (e.g. Patent Literatures 1 to 3). Such a reflector has needs such as suppression of change in a shape of an optical surface, improvement in adhesiveness of the reflection film and improvement in corrosion resistance of the reflection film. Especially for a reflector placed under a severe usage environment like an on-vehicle product, extremely high environmental resistance is required.
Patent Literature 1 describes a light reflector in which a reflection film containing silver is formed on a surface of a plastic substrate, wherein the plastic substrate is a heat curable resin formed article, an adhesiveness-improving film is made of Cr, Al2O3, LaTiO3 and the like, and a reflection-enhancing layer is made of a multilayer of Y2O3, Al2O3 and the like. Here, each film constituting the reflection film is formed by vapor deposition using plasma.
Patent Literature 2 describes a reflector having a structure that a film mainly composed of aluminum oxide is formed directly on both sides of a silver film, wherein both the films mainly composed of aluminum oxide on both sides are made of aluminum oxide or aluminum oxynitride. Here, both the films mainly composed of aluminum oxide are formed by a sputtering method, an ion beam-assisted deposition method, or the like.
Patent Literature 3 describes an optical mirror for laser beam having a substrate layer, a stress-adjusting film formed on the substrate layer to deform the substrate layer into a convex shape, and a reflection film formed on the stress-adjusting film, wherein the stress-adjusting film contains any of zinc sulfide, cerium oxide and silicon oxide. Here, this stress-adjusting film is formed by physical assistance including ion or plasma.
Patent Literature 1 describes resistance of the reflection film to the environment, but a concrete method or quantitative explanation is not given, e.g. it is not specifically described by what configuration and to what extent the high durability can be achieved.
In Patent Literature 2, although the reflectance is improved by a heat treatment at 50 to 200° C., only the change in reflectance at a temperature of 70° C. and a humidity of 90% is measured for the conditions including moisture resistance, and a specific method for improving moisture resistance is not disclosed.
Patent Literature 3 discloses a structure that deformation of the convex surface is suppressed by the stress-adjusting film having a tensile stress which countervails a tensile stress of an Ag film, but it does not disclose durability under a hot environment and a hot and humid environment.